1. Field of the Invention
The present invention relates to an optical disk apparatus for reproducing information recorded on an optical disk or for recording information on an optical disk and reproducing the information recorded on the optical disk, and a tilt control method of the optical disk apparatus.
2. Description of the Related Art
Control, specifically called tilt control is carried out in an optical disk apparatus such as a video recorder, a video camera or a PC optical storage disk drive, so as to read information from or write information on an optical disk such as a CD (Compact Disc), a DVD (Digital Versatile Disc) or a BD (Blu-ray Disc). The tilt control is control for correcting coma aberration which occurs when the laser beam axis of an optical pickup is not kept perpendicular to the recording surface of the optical disk. By the tilt control, the optical disk apparatus can record and read information properly in spite of surface fluctuation of the optical disk which is rotating, a warp in the optical disk, or the like. Thus, the optical disk apparatus can perform recording/reproducing stably.
There is a known technique for tilt control (for example, JP-A-2004-95044) as follows. That is, a DC voltage value is obtained in each of an inner circumferential position and an outer circumferential position of the optical disk based on a driving signal to be supplied to a focusing coil built in the optical pickup. A relation between each position and the DC voltage value is obtained from the obtained DC voltage value. A signal obtained by adding a DC voltage value calculated from the obtained relation to an AC signal contained in the driving signal to be supplied to the focusing coil is supplied to a tilt adjustment coil so as to adjust the inclination of an objective lens.
A technique for adjusting inclination of an objective lens with respect to the surface fluctuation of an optical disk has been known as described previously. However, the technique cannot be applied to an optical disk apparatus directly. This has something to do with a difference in configuration between a focus control system and a tilt control system, in that the former is based on feed-back control using an error signal, while the latter is based on feed-forward control. A signal frequency band required in the tile control system is as wide as a disk rotation period or about twice as large as the disk rotation period in consideration of a warp of the disk or the like. In frequency, the band is several tens to several hundreds of Hz. Since the tilt control system is based on feed-forward control, a control command value corresponds to a control set value, and its gain is 0 dB. On the other hand, in the focus control system, an error with respect to a set value is suppressed sufficiently, and the gain reaches several tens of dB in the frequency corresponding to the disk rotation period. The signal band of the focus control system reaches about several kHz. Thus, when a signal in the focus control system is applied directly to the tilt control system so as to track the surface fluctuation of the optical disk, the signal is too rich in unnecessary high-frequency components to be practical in view of the signal quality for tilt control. Thus, there is a problem that servo deviation is caused by resonance of an actuator or the like.
If configuration is made in such manner that the signal is passed through a low pass filter to limit the high-frequency signal band of the focus control system which band will be unnecessary for tilt control, the signal quality can be improved. However, a phase delay occurs due to the insertion of the low pass filter. Thus, the signal has a certain amount of delay with respect to the surface fluctuation of the optical disk. When the signal quality is improved in this manner, the influence of a phase angle cannot be ignored. Thus, there is a problem that the tilt control cannot track the surface fluctuation of the optical disk accurately.